Technical Field
The present invention relates to a position measuring apparatus. In particular, the present invention relates to a position measuring apparatus suitable for measuring the position of a workpiece, which is a measurement target, with high accuracy in a numerically controlled (NC) machine tool and the like.
Background Art
In a machine tool apparatus such as a NC machine tool or a NC lathe, it is necessary to accurately measure the position of a jig or a measurement target, which is a center, in the apparatus. Typically, such an apparatus has a measurement target as well as a main spindle or a turret that has attached thereto a stylus to touch the target. The apparatus also has a means for monitoring the load (load torque) of a feed motor for the main spindle or the turret. Such a load monitoring means is adapted to detect fluctuations (increase) in the load torque at a moment when the stylus touched the target and capture the position information on the stylus at that moment as the position information on the measurement target. Such a scheme is typically called a load monitoring scheme, and an example thereof is disclosed in Patent Document 1, for example. Herein, when the value of current (load) of a feed motor exceeds a predetermined value, it is basically determined that a moving body (stylus) touched a stationary object (measurement target), and thus, position information at that time is recorded.